A molecular level understanding of how key signaling proteins distinguish between nitric oxide (NO) and oxygen is the central focus of this proposal. NO is a key signaling molecule that plays a central role in blood vessel dilation, penile erections and other smooth muscle related responses. NO also is a signaling agent in the central nervous system and is used by the immune system to kill infectious organisms and tumor cells. NO is toxic and chemically reactive and major questions remain concerning how this molecule is used with specificty in humans and other animals. Recent observations clearly show that bacteria contain a family of proteins that are closely related to the enzyme guanylate cyclase, the NO receptor (sensor) in humans. However, while in some cases our hypothesis is that these prokaryotic proteins are involved in NO sensing, in others is appears that they are O2 sensors. How are these proteins able to distinguish NO from oxygen is the broad overall goal of this proposal as well as understanding function. The results obtained are likely to explain not only how NO and oxygen are sensed in bacteria but will also shed light on how these molecules are sensed and used in humans and other animals. Further, the work will provide novel data on why some bacterial pathogens are not killed by NO. Experimentally this project will involve general tools of molecular biology and protein characterization plus advanced spectroscopic techniques such as resonance Raman spectroscopy. In addition, protein crystallization structure determination and functional studies by gene transfection into E. coli will be used. The structural determinants that allow for this difficult discrimination are not obvious and remain at the heart of biological recognition and specificity of signaling. What appeared for the last 10 years to be a characterization of proteins that bind NO has now been expanded to include oxygen. CO signaling is an active area of research lacking among other things a specific receptor. Proteins in this family are likely possibilities for this key, missing component. Furthermore, when pathogens respond to a NO challenge from the immune system, they could use a receptor system similar to the apparent NO signaling system hypothesized to be contained in a number of facultative aerobic bacteria.